1. Field of the Invention
This invention relates to processing methods for memory devices, such as dynamic random access memory. In particular, this invention relates to processing methods that leverage tools and techniques available in the semiconductor industry for the production of molecular memory cells, molecular memory arrays, and electronic devices including molecular memory.
2. Relevant Background
Conventional CPUs, memory devices and data communication mechanisms are mass produced as solid state electronic devices. Although sometimes referred to as “semiconductor devices”, solid state electronic devices rely on electrical behavior of solid materials including metals, semiconductors, and insulators. The techniques and equipment for producing solid state devices have improved dramatically over time to enable the production of devices such as switches, capacitors, resistors, and interconnections with sub-micron scale features at higher yields and lower cost.
Advances in semiconductor processing and device design have resulted in memory devices, for example, that implement hundreds of megabits of storage on a single integrated circuit. Such devices include volatile memory such as dynamic random access memory (DRAM) and static random access memory (SRAM), and non-volatile memory such as electrically erasable programmable read only memory (EEPROM), Flash RAM, ferroelectric DRAM, and the like. Memory manufacturing processes continue to push the limits of fine-geometry patterning and machining technology.
However, physical limitations on the materials and tools used to manufacture solid state electronic devices will not always support making smaller and smaller devices. Even where smaller geometries can be implemented, device performance may suffer. For example, memory devices with smaller storage capacitors require more frequent refreshing so that the power and time consumed by refresh processes limit overall device performance and can also increase soft error rates. Moreover, the capital and intellectual investment required to continue improving materials, processes and tools so that smaller geometry components can be manufactured is a burden on future development in the solid state manufacturing industries. As a result, alternative technologies for manufacturing computing devices and particularly memory devices are being considered.
Another problem facing memory designers trying to increase information density (e.g., the amount of information that can be stored in a given area of the memory chip) is that each memory cell of a conventional solid state capacitor can only store one bit of information. Accordingly, it would be desirable to have processes for manufacturing memory devices with improved information storage density achieved by having a memory cell that can reliably store a plurality of discrete states.
One area of investigation includes molecular devices that implement some or all components of an electronic device or system with molecular scale structures and components. These molecular scale structures and components exhibit molecular rather than solid state behavior, which can provide enhanced performance in many instances. Moreover, because molecules retain their essential properties down to the individual molecule level, molecular-scale components and device structure can be scaled (or shrunk) as future processing tools and technology are developed.
The approach of using molecules in electronic devices such as switches, capacitors, conductors and the like, depends on the development of attachment chemistries and processes to achieve high yield at reasonable throughputs and costs. Because current technology relies on physical patterning of device structures, chemical approaches to electronic device manufacture have not been used in production environments. To be certain, existing processes in the semiconductor industry rely heavily on a variety of chemical processes, however, these chemical processes are used to deposit, etch, shape, clean and modify materials that make up the devices. The chemicals themselves are rarely left on the finished device, except as contaminants, and are not used to form active device structures.
As such, equipment vendors and tool development engineers have not designed process tools to apply efficient attachment chemistries that can be used in molecular electronic device manufacture. Molecular scale components require repeatable processes that are able to attach desired chemical species to substrates, other device structures, and each other. Robust processes for forming molecular structures enable new types of components such as electrochemical cells to be implemented with semiconductor devices.
While it is desirable for molecular manufacturing techniques to be compatible with existing semiconductor industry processes and to use existing semiconductor industry tools, molecular device structures are sensitive to many variables and conditions that do not trouble semiconductor processes. For example, water is present throughout most semiconductor manufacturing processes as a cleaning fluid and in the form of ambient humidity. However, water can have destructive effects on some molecular processes as water molecules interfere with the attachment chemistry or destroy the active molecules. Similarly, thin native oxide layers and ultra-low contaminant levels are tolerable in semiconductor processes because the bulk effects of these aberrant features are minimal in comparison to the overall device function. In contrast, when devices are manufactured with molecular-scale features, these molecular-scale defects can become significant.
In view of the above, it is apparent that a need exists for processes for manufacturing molecular memory cells, molecular memory arrays, and electronic devices including molecular memory. Further, there is a need for molecular memory devices that can be manufactured using techniques that are compatible with existing semiconductor manufacturing practices so that semiconductor devices and interconnections can be manufactured monolithically with molecular memory devices.